The present invention relates to a transistor circuit, and more particularly, to a transistor circuit for switching a write current for magnetic recording.
In a conventional transistor circuit for switching a write current for magnetic recording, there is known a circuit configuration shown in FIG. 3. In FIG. 3, the conventional circuit includes a current mirror circuit portion and a current switching circuit portion.
The current mirror circuit portion includes an NPN transistor Q.sub.31 having its collector connected to an input terminal IN.sub.31 and its emitter connected to a resistor R.sub.31 connected to the ground GND, and another NPN transistor Q.sub.32 the base of which is connected to the collector of the NPN transistor Q.sub.31. The base of the NPN transistor Q.sub.31, the emitter of the NPN transistor Q.sub.32 and the base of an NPN transistor Q.sub.33 are connected together and are connected through a resistor R.sub.32, to a ground terminal GND. The collector of the NPN transistor Q.sub.32 is connected to a power supply terminal Vcc. The emitter of the NPN transistor Q.sub.33 is connected to the ground terminal GND through a resistor R.sub.33, and the collector of the transistor Q.sub.33 constitutes an output terminal of the current mirror circuit portion.
The current switching circuit portion includes a pair of NPN transistors Q.sub.34 and Q.sub.35, the emitters of which are commonly connected to the output terminal of the abovementioned current mirror circuit portion, which is formed by the collector of the NPN transistor Q.sub.33. The base and the collector of the NPN transistor Q.sub.34 are connected to a control terminal C.sub.31 and an output terminal O.sub.31, respectively. The base and the collector of the NPN transistor Q.sub.35 are connected to another control terminal C.sub.32 and another output terminal O.sub.32, respectively. Thus, all the transistors in the circuit of FIG. 3 are of an NPN conductivity type, for the purpose of illustration. However, all of those transistors may be of a PNP conductivity type in operation.
The operation of the conventional circuit will now be described.
When an input current I.sub.I31 is applied from the input terminal IN.sub.31, the current mirror circuit portion inverts the input current to produce an output current I.sub.O3. The output current I.sub.O3 is drawn in or fed into the collector of the transistor Q.sub.33. The magnitude of the output current I.sub.O3 is determined by the emitter size ratio of the transistors Q.sub.31 and Q.sub.33 and/or the resistance ratio of the resistors R.sub.32 and R.sub.33.
The current inverted by the current mirror circuit portion is outputted at the output terminal O.sub.31 or O.sub.32 or is switched therebetween by the current switching circuit portion. When the potential at the control terminal C.sub.31 is higher by above 100 mV than the potential at the control terminal C.sub.32, a current is drawn in from the output terminal O.sub.31 as an output current I.sub.O31. On the other hand, when the potential at the control terminal C.sub.32 is higher by above 100 mV than the potential at the control terminal C.sub.31, a current is drawn in from the output terminal O.sub.32 as an output current I.sub.O32. Further, when the potential difference between the control terminal C.sub.31, C.sub.32 and the ground terminal GND is smaller than 0.7 V, a current is not drawn into the transistor Q.sub.33 from either of the output terminals O.sub.31 and O.sub.32.
The conventional magnetic recording write current switching circuit as mentioned above has the following three problems:
Firstly, bipolar transistors having a finite current amplification factor h.sub.FE are used for current switching. The h.sub.FE values of transistors formed in an integrated circuit (IC) are uneven, and as a result, an error in the base current component is caused, thereby making it difficult to provide output currents with high accuracy. The accuracy of write current for magnetic recording is usually in the order of .+-.5%. For example, assume that the current amplification factor h.sub.FE has a fluctuation of 50 to 200 in manufacture, the output current I.sub.O31 or I.sub.O32 fluctuates between 98.0% and 99.5% of the output current I.sub.O3. Therefore, the output current results in an unevenness or fluctuation of 1.5% in its value due to the unevenness of the current amplification factor h.sub.FE.
Secondly, when the potential difference between the control terminal C.sub.31 and the ground terminal GND and the potential difference between the control terminal C.sub.32 and CND both are set to be smaller than 0.7 V in order to prevent a current from being drawn in from the output terminals O.sub.31 and/or O.sub.32, the bipolar transistor Q.sub.33 causes a saturation, so that if the current switching circuit is formed in a monolithic integrated circuit, a large amount of current passes through a substrate in which the monolithic integrated circuit is formed due to an action of a parasitic transistor of the transistor Q.sub.33, to a disadvantage.
Thirdly, the potential at the output terminals O.sub.31, O.sub.32 must be kept at a high value in order to improve the accuracy of the output current. However, when the potential at the output terminal O.sub.31 or O.sub.32 falls due to a flyback voltage generated at the switching of a magnetic recording write current, or when the potential at the output terminal O.sub.31 or O.sub.32 is low, the output current I.sub.O31 or I.sub.O32 is not drawn into the transistor Q.sub.33. As a result, the transition time for current switching becomes elongated, to disadvantage. More specifically, when the potential at the control terminal C.sub.31 is higher than the potential at the control terminal C.sub.32, the potential at the output terminal O.sub.31 must be kept at a voltage (in the order of 0.5 V) higher than the emitter potential of the transistor Q.sub.34 and higher than the saturation voltage of the transistor Q.sub.33. In other words, the potential at the output terminal O.sub.31 must be kept at a voltage larger than the potential at the control terminal C.sub.31 minus 0.2 V. The minimum potential at the control terminal C.sub.31 is determined to be 1.5 volts since the voltage drop across the resistor R.sub.33 is about 0.3 V, the saturation voltage of the transistor Q.sub.33 is about 0.5 V and the base-emitter voltage of the transistor Q.sub.35 is about 0.7 V. Accordingly, the minimum potential at the output terminal O.sub.31 has to be 1.3 V. Thus, the conventional current switching bipolar transistor circuit does not operate at a lower potential at which the output terminal is kept.